Cannot the user  simply change the value in a VBA code module?
That would be the simplest solution.

Where is the user  changing the rate? That is where is it being set?

Another possible way to go is to ask for the value when the workbook is
opened maybe

My intention was to have a button in the spreadsheet, accessed by
only authorised people, who are able to change the 'Default Exchange
Rate'.
I usualy keep this type of info in a separate sheet as 'Hutch' has
suggested, but I am trying to maximise the amount of
data/functionality into the codeing and thus minimise the size of the
spreadsheet.

A separate sheet with all these
constants, including other things like file and folder locations. It is the
most elegant way to optimise your coding (assuming all the locations have
Range Names!)

Thanks all, I have decided to keep the constants on a separate sheet.

IMHO, saving the value in a worksheet is by far the easiest and best approach.
The ability to do that is one of the nice things about programming in Excel. You
could save  the exchange  rate on another sheet which you can set  to xlVeryHidden
and password protect.
- I don't recommend having a user  open the VBE and edit code (even if it's
just a Constant). - Another option is to write the data out to another file,
such as a text file or another Excel workbook. The next time  your application
workbook is opened, your code can look for the other file to retrieve the
exchange rate. Hopefully the other file will still be there.

Or use an Excel name to store it. You can even hide the name if wished.